An optical fiber (or fiber) is a special type of waveguide that is typically manufactured from glass, such as silica. A fiber includes a core that operates as a waveguide for the transmission of light. The core is surrounded by one or more layers of a medium, known as a cladding. Usually, the core is located at the center of the fiber surrounded by the cladding, although a fiber can be provided with an off-center core, such as in some double-clad fibers. Most fibers, including those used in laser optics, typically have a core with a refractive index that is higher than that of the cladding. The increased refractive index of the core can be obtained by doping the glass core with an index-raising material, such as germanium. The refractive index contrast between core and cladding determines the numerical aperture of the fiber.
A fiber amplifier typically includes a gain fiber having a core that includes rare-earth dopant ions. For example, the core of a fiber can be doped with laser-active ions, such as rare-earth ions of erbium, neodymium, ytterbium, or thulium. One or more laser diodes (or other pumping means) are coupled to the fiber to provide a pump signal to the fiber core. When optically pumped, the fiber exhibits gain over a wavelength region that is characteristic of the rare-earth dopant. The amplifier gain is related to the amount of pump power coupled to the gain fiber as well as to the length of the fiber.
Most fibers operate within expected parameters at low power levels, such as are employed for most telecommunications applications. As the power through an optical fiber increases, however, problems can develop which may decrease performance as well as damage the fiber and/or surrounding components. As one example, a significant amount of light may be guided in the cladding of the optical fiber, which can interfere with the signal in the core at the output of the fiber. One approach for separating the cladding light from the core light employs free-space filtering, where the output of the fiber is allowed to propagate in free space over many meters. The higher diverging cladding light spatially separates from the lower diverging core light the further the beam is allowed to propagate. This approach is good for measuring the raw power of the light from the fiber's core, but it usually is ineffective in sufficiently removing all cladding light from the core light thereby causing problems for more subtle applications. This approach also typically requires significant bench top space to implement.